The present invention pertains generally to plasma enhanced bonding of metallic materials, and more particularly, to a method and apparatus for using hot plasma gas to simultaneously heat, clean, and activate the bonding of surfaces of metals both immediately before bonding and as the surfaces are fused together.
Wire bonding is a technique used in the semiconductor industry to connect different integrated circuit (IC) pads together via wires. Current wire bonding methods fall into one of three predominant categories: thermocompression, ultrasonic, and thermosonic bonding. Each wire bonding method involves a three-step process to bond the wire to a surface. The first step involves that application of force which impels the atoms of the metal wire to within a close enough distance to the atoms of the pad to allow diffusion bonding to take place, and also deforms the metal wire against the pad to create a larger bonding surface, resulting in a larger bonding footprint. The second step involves the application of bonding energy to the atoms of the wire and pad. The bonding energy is thermal energy when using thermocompression, ultrasonic energy when using the ultrasonic bond method, and a combination of thermal and ultrasonic energies when using the thermosonic methods. The third step involves the passage of an appropriate amount of time to allow the two surfaces to fuse together.
Faster bonding has not heretofore been achieved due to the inability to prevent contaminants from adhering to the bond site surfaces. Surface contamination is known by experts in the wire bonding field to cause poor wire bond adhesion, resulting in less robust bonds and therefore less reliable interfaces. Accordingly, much effort has gone into cleansing the surfaces of the metals to be wire bonded. Current wire bonding processes now include a plasma bond pad cleaning prior to wire bonding in order to enhance the bondability of the wire bond surfaces. Plasma cleaning is a technique involving the use of gas plasma to remove organic contaminants from surfaces. By removing surface contamination, plasma cleaning increases the bonding or adhesive properties of the bondsite surface.
A plasma is a collection of positive, negative, and neutral particles in which the density of the negatively charged particles is equal to the density of the positively charged particles. When an energetic electron strikes a neutral gas molecule, it can cause dissociation and form free radicals and ions. The free radicals cause chemical reactions for destroying contaminants. For example, with oxygen, the dissociation process produces the free radical atomic oxygen (O). This reactive species has enough energy to break a carbon-carbon bond.
Prior art plasma cleaning involves placing the pieces to be bonded into a plasma chamber, removing air from the chamber to create a vacuum, introducing a gas or gaseous mixture into the chamber, and applying energy to the chamber to produce the plasma. In the presence of the plasma, organic contaminants on the bondsite surfaces are converted to carbon monoxide, carbon dioxide, water vapor, and/or other gasses, which are pulled out of the plasma chamber by a vacuum pump. After a suitable amount of time, the gas flow and energy are shut off, and the chamber is then purged with a nonreactive gas, such as nitrogen, to remove all traces of volatile compounds. Finally, the chamber is returned to atmospheric pressure. A cleaning cycle usually lasts from between 30 seconds to 15 minutes and is largely a function of the workpiece loading in the plasma chamber.
The current methodologies for plasma cleaning of wire bond surfaces is problematic. First, as soon as the components exit the cleaning chamber, they are immediately exposed to surface recontamination due to the organic particles in the air. In addition, the separate plasma cleaning chamber and equipment, coupled with the long amount of time required to set up and execute the cleaning process, is quite costly. Accordingly, it would be desirable to have a method and apparatus that allows expedient plasma cleaning of wire bond surfaces without the use of a separate expensive cleaning chamber.
The present invention is a novel method and apparatus for performing plasma cleaning of wire bond surfaces at the time the bonding occurs. The invention effectively generates a plasma bath, or xe2x80x9cbubblexe2x80x9d, around the bond site which performs the simultaneous functions of heating, cleaning, and activating the bonds of the surfaces of the metals that are to be fused together.
In accordance with the method of the invention, an energizing circuit ionizes a gas to form a plasma, which is directed in a directional pressurized flow at the wire and bonding site surface to form a dynamic plasma cleaning chamber bubble around the portion of the wire and the bonding site surface that are to be bonded together. The dynamic plasma cleaning chamber bubble is maintained at the bond site prior to and during the actual bonding of the surfaces in order to clean and prevent recontamination of the site during the bonding process.
In one embodiment, the plasma enhanced wire bonder in accordance with the invention includes a bonding device that performs the bonding of the wire to the bond site surface. The bonder also includes a plasma gun which applies a pressurized directional flow of plasma directed at the wire, the bond site surface, and the portion of the bonding device that comes in contact with either the wire or bond site surface. Preferably, an energizing circuit is included to energize the gas supplied to the plasma gun to generate the plasma. In the preferred embodiment, the gas is argon or a mixture of argon and oxygen. The gas is energized via direct current voltage or high frequency alternating current (i.e., radio frequency, RF). Preferably, the plasma enhanced wire bonder also includes a temperature detector which senses the temperature of the bond site, and a controller which adjusts the energizing circuit to adjust the plasma temperature to maintain the bond site temperature within a predetermined temperature range.
The present invention describes a novel technique that cleans the metal surfaces as they are bonded to eliminate any contaminants from being introduced into the final bond, thereby improving the wire bond adhesion. The invention may also allow materials to be bonded together that prior to the present invention were difficult to bond due to compounds formed at the bond site, which resulted in weaker bonds. In addition to the above-named advantages, the temperature of the plasma/hot gas is high enough that the bonding device (e.g., wedge or ball bonder) need not include a heater. The invention essentially provides xe2x80x9clocalizedxe2x80x9d heating within the area of the dynamic plasma cleaning chamber bubble, which reduces the risk of accidentally unsoldering other wire bonds on the device being bonded.